||This is happening to me right now for the first time. I don't have a recipe to reproduce it, of course, and it will probably go away tomorrow, but I wanted to let you know in case it is useful.
I did about a hundred problems today, and currently the main page says I have 0 New problems, 0, Learning problems, and 1 Review problem. When I click on Review, though, I just come back to the main page, still saying that I have 1 Review problem left. The Scheduled Problems graph on the stats page says I don't have any problems left for today (the first bar is for tomorrow).